Support Elixir 1.15

[scottming]

Fixes #101
Closes #243

Starting with Elixir 1.15, we don't need to capture io for compiling individual files or quoted anymore,
because with_diagnostics returns all compile-time diagnostics.

So I have implemented two ways to compile and process the compiled results.

You can see from the tests that the location of the undefined diagnostic is more precise,
and I've also enhanced the unused and undefined messages to make them more concise and clear.

Showcase

Undefined

Unused

Known issues

• NAMESPACE works very well, but mix release lexical won't work on lexical project. Fixed by 1.15.4
• compile twice. no idea about this issue, because mix deps.compile in command line has this problem too. Fixed by 1.15.4
• We should wait until 1.15.3, because there are some bugs in 1.15.2, Elixir core team already fixed them: Clarify that Code.with_diagnostics/2 does not rescue elixir-lang/elixir#12742 (comment).

[scohen]

Looking pretty good.
I don't like the if statements in the tests, but I don't know of a better way to do it.

Just a couple comments.

[scohen]

Thought: There is a lot of version checking sprinkled throughout the codebase now, I don't like that. I think instead, we should be checking for features, or at least indicate that we care about some feature being present.

To that end, I suggest we write a Features module in common that's something like:

defmodule Elixir.Features do 

  def with_diagnostics? do 
     Version.match?(System.version(), ">= 1.15.3")
  end
end

Then delegate to that. This also makes testing easier because you can patch that module to turn off a specific feature.

[scottming]

To that end, I suggest we write a Features module in common that's something like:
Good idea.

[scohen]

flagging for a follow up commit: compile_keeps_current_directory?